1. Field of the Invention
The present invention relates to a method of and a device for displaying distribution patterns of frequency-division multiplexed signals which have been received and equalized.
2. Description of the Related Art
In data transmission among data processor terminals using analog lines, a sending terminal sends modulated digital data onto a transmission line, and a receiving terminal demodulates the received signal to recover the original digital data. The equipment for implementing this type of data transmission is called a modulator/demodulator (modem).
Data multiplex transmission systems include time-division multiplexing (TDM) and frequency-division multiplexing (FDM). The FDM systems include a system provided with a secondary channel used for monitoring the network by transmitting signal quality information, error rates, etc., in addition to a main channel used for transmitting main data, a system which divides the main channel into more than one channel, and so on.
In the time-division multiplexing system, a single carrier frequency is used. Thus, distribution patterns (the distribution of received signals on a two-dimensional plane) of equalized output signals on time-division channels obtained by a demodulator in a modem are essentially the same.
In contrast, there is more than one carrier frequency in the frequency-division multiplexing system. Distortion caused by a transmission line differs among frequency channels. Thus, the distribution patterns of equalized output signals obtained from the demodulator in a modem differ among channels. For this reason, in order to monitor the receive conditions of signals transmitted on a frequency-division multiplexing basis, it is necessary to examine a distribution pattern for each channel.
Heretofore, in examining a distribution pattern on each channel, receive channels are selected in sequence to display a distribution pattern for each channel.
FIG. 1 is a diagram used to explain a conventional method of displaying distribution patterns. In this example, a voice band of 0.3 to 3.4 KHz is divided, as shown in FIG. 1A, into four channels; an A channel, a B channel and a C channel for data transmission, and a secondary channel over which signal quality information is transmitted.
When monitoring the receive condition of each of the channels assigned to corresponding frequency bands, a user selects the channels in sequence in such a way as to select the A channel first to display the distribution pattern of an A-channel equalized output signal (FIG. 1B), the B channel second to display the distribution pattern of a B-channel equalized output signal (FIG. 1C), the C channel third to display the distribution pattern of a C-channel equalized output signal (FIG. 1D), and the secondary channel finally to display the distribution pattern of a secondary-channel equalized output signal (FIG. 1E).
That is, with a modem which modulates and demodulates frequency-division multiplexed signals, it is necessary to observe the distribution pattern of each of frequency-divided channels, thereby understanding the receive condition of each channel. However, with the conventional distribution pattern displaying method, since only a single distribution pattern at a time is displayed, in order to display the distribution pattern of each channel and understand the receive state of each channel, the user is required to perform switching from one channel to another. This operation is troublesome, and has the problem that the entire receive condition cannot be recognized simultaneously.